1876.] 



Magnetic Dip fyc. at Kew Observatory. 

 Table II. 



235 



Date. 



Corrections 



for secular 



change. 



67° 54'-82 



+ secular 



change. 



Observed 

 values. 



Observed minus calculated. 



April to 

 September. 



October to 

 March. 



July 1, 1869 



+ 5-91 



68 0-73 



68 052 



-0-21 



i 



Jan. 1, 1870 



4-4-84 



67 59-76 



0-30 





-f 0-64 



July 1, 1870 



+ 376 



58-58 



67 57-78 



-0-80 





Jan. 1, 1871 



+ 2-69 



57-51 



58-18 





+ 0-67 



July 1, 1871 



+ 1-61 



56-43 



56-48 



+ 005 





Jan. 1, 1872 



+ 0-54 



55-36 



55-77 





+ 0-41 



July 1, 1872 



-0-54 



54-28 



53-45 



-0-83 





Jan. 1, 1873 



-1-61 



53-21 



53-75 





4-0-54 



July 1, 1873 



-2-69 



52-13 



51-65 



-0-48 





Jan. 1, 1874 



-3-76 



51-06 



50-73 





-0-33 



July 1, 1874 



-4-84 



49-98 



49-90 



-008 





Jan. 1, 1875 



-591 



4891 



49-38 





4-0-47 



-0-39 



4-0-40 



These six years show a semiannual inequality in the dip, the mean 

 giving a lower reading by 0'*40 in the six months from April to September, 

 and a higher by 0'*40 in the other six monthis, viz. October to March, 

 than the average for the year. 



This amount appears to be variable, the range in the six years ending 

 March 1863 published by Sir E. Sabine being l'-31, in the six years 

 ending March 1869 discussed by Dr. Stewart but r, 54, and in the six 

 years now under consideration 0'*80. 



The mean annular secular change as deduced from the three series of 

 observations is as follows : — 



Epoch. 



Mean dip. 



Difference. 



Mean annual 

 secular change. 



April 1, 1860 



68 20-07 

 68 6-62 

 67 54-82 



13-45 

 11-80 



2-26 

 1-97 



April 1, 1866 



April 1, 1872 





Taking the mean secular change from the value for each year, we have 



Series 1 2- 00 



„ 2. 1-92 



„ 3 2-15 



The probable error of a monthly determination of the dip as found by 

 the method of least squares from the six years' observations, after the 

 application to each observation of the correction for secular and semi- 

 annual change, is + 0' # 78. 



The error in series 1 was ± 0'-71, and in series 2 4- 0'-96. 



